Newly-designed wireless system uses 2-way radio wave communication for enhanced communication among master and slave units. Achieves a transmission distance of up to 98.4 ft./30 m, all at a 360 degree angle.

The Speedlite Transmitter ST-E3-RT facilitates radio controlled, two-way wireless transmission up to 98.4 feet, for a reliable and robust connection among master and slave units. Communicating on 2.4 GHz frequencies, the Speedlite Transmitter ST-E3-RT does not have the same directional limitations of traditional wireless optical transmitters. Where other wireless signals can be interrupted by physical obstacles, radio controlled systems excel. Fifteen transmission channels are available, selectable manually or automatically, and radio transmission IDs can be set to prevent misfiring in the event of signal interference on the same channel. With the extended, more reliable range afforded by radio transmission, ambitious photography is made easier.

Great radio, when there's ambient light
I used the ST-E3-RT with 3 600EX-RT flashes for a wedding reception and loved the ease of the system and the results -- that is, when I could focus.
There is no focus assist beam when the radio trigger is on-camera, so as soon as it gets dark, focus performance diminishes significantly. You then have to mount one of the flashes on-camera to get improved focus, which defeats the whole purpose of off camera flash.
So, the radio system works best when you're trying to create some fill in good-to-average light situations. Once it gets dark and you want to use the system to create off camera light, you're out of luck.
April 28, 2014

90 % perfect
This is a product I have wanted for a long time, and it works very well, but......
It should articulate like the flashes do, meaning go from the flat position that is currently to a screen up, vertical position so it can be read when the camera is at an eye level position on a tripod. I would like to see this change made in a new product. Very frustrating to have to either remove it from camera or remove whole camera from tripod to change settings, or carry a small ladder to be able to see the screen while it is mounted to a tripod. Just my 2 cents. I own a lot of canon gear including four (4) 600 ex rt speedlights which I love. Make this product go from good to GREAT! You can do it.....
February 23, 2014

Where's the AF assist beam?
Everything works great except No AF Assist Beam. Need that beam, put that in asap. The main flash 600EX-RT must be mounted on the camera at all time at night time, so what's the point of buying this ST-E3-RT then?
December 14, 2013

Great addition
Also have a single 600 flash. Wish they had an adaptor for my 580 flash. But great for getting rid of the cord for macro, hand pointing the flash, remote location of flash. I will be adding at least 1 more 600 flash to the arsenal.
October 11, 2012